Functional and Morphological Adaptations in the Heart of Children Aged 12–14 Years following Two Different Endurance Training Protocols

s study investigated the cardiac functional and the morphological adaptations because of two endurance training protocols. Untrained children (N = 30, age: 12–14 years) were divided into three groups (N = 10/group). The rst group did not perform any session (CONTROL), the second performed ventilatory threshold endurance training (VTT) for 12 weeks (2 sessions/week) at an intensity corresponding to the ventilatory threshold (VT) and the third (IT) performed two sessions per week at 120% of maximal oxygen uptake (VO 2max). Two other sessions (30 min running at 55–65% of VO 2max) per week were performed in VVT and IT. Echocardiograms (Left Ventricular end Diastolic Diameter, LVEDd; Left Ventricular end Diastolic Volume, LVEDV; Stroke Volume, SV; Ejection Fraction, EF; Posterior Wall Thickness of the Left Ventricle, PWTLV) and cardiopulmonary ergospirometry (VO 2max, VT, velocity at VO2max (vVO 2max), time in vVO 2max until exhaustion (Tlim) was conducted before and after protocols. Signi cant increases were observed in both training groups in LVEDd (VTT = 5%; IT = 3.64%), in LVEDV (VTT = 23.7%; ITT = 13.6%), in SV (VTT = 25%; IT = 16.9%) but not

PWTLV) and cardiopulmonary ergospirometry (VO 2max, VT, velocity at VO2max (vVO 2max), time in vVO 2max until exhaustion (Tlim) was conducted before and after protocols. Signi cant increases were observed in both training groups in LVEDd (VTT = 5%; IT = 3.64%), in LVEDV (VTT = 23.7%; ITT = 13.6%), in SV (VTT = 25%; IT = 16.9%) but not in PWTLV and EF, after protocols. No differences were noted in the CONTROL group. VO2max and VT increased signi cantly in both training groups by approximately 9% after training. Our results indicate that intensity endurance training does not induce meaningful functional and morphological perturbations in the hearts of children. Keywords:exercise; cardiac; track and eld; athlete; anaerobic threshold; VO 2max 1. Introduction It is well-known that physical training is associated with hemodynamic changes that bring about morphological and functional adaptations in the heart of athletes [1–5]. The increase in the heart mass of athletes was rst observed by Henschen and Skidlauf in 1899 [6] through a percussion-based examination of the thorax, and was later con rmed through radiography and proven through autopsy. The advent of echocardiography allowed researchers to obtain a more detailed insight into the adaptations that a heart undergoes through exercise, while the use of magnetic resonance imaging (MRI) in recent years has further helped elucidate these adaptations. Although the existence of an “athletic heart” is nowadays commonly accepted among researchers, the Morganroth theory [6], which prevailed for many years, is veri ed only in part, since the concentric cardiac adaptation of dynamic-sport athletes and the non-increase in the wall thickness of the left ventricle (LV) in endurance athletes of strenuous sports are not veri ed by more recent research [7–9]. Concentric hypertrophy is associated with increased left ventricular wall thickness without overall enlargement and increased Left Ventricular end Diastolic Diameter (LVEDd), whereas eccentric hypertrophy is characterized by dilatation of the ventricular chamber, accompanied by symmetrically increased wall thickness. All athlete Sports2023,11, 157.

wall thickness without overall enlargement and increased Left Ventricular end Diastolic Diameter (LVEDd), whereas eccentric hypertrophy is characterized by dilatation of the ventricular chamber, accompanied by symmetrically increased wall thickness. All athlete Sports2023,11, 157.

Sports2023,11, 157 2 of 13 categories show an increase in LVEDd and the Posterior Wall Thickness of the Left Ventricle (PWTLV), which is analogous. However, in endurance sports, cardiac hypertrophy is most evident with long-distance athletes exhibiting extremely high values in LVEDd [3,10–12]. The literature contains several studies examining the effect of endurance training on the morphology and function of the adult heart [9,13–17], but few have addressed the effects of endurance training on the hearts of children and adolescents [18–23]. Previous studies on preadolescent (11–13 years of age) distance runners indicated that endurance training does not result in signi cant changes in cardiac remodeling [24], whereas ve months of swimming training resulted in signi cant remodeling of the right ventricle of competitive athletes [25]. A comparison between preadolescent cross country skiers and non-competing individuals revealed that adaptive remodeling can occur in this age since athletes had greater left ventricular mass and greater left and right ventricular chamber dimensions [26]. Furthermore, a shift in the development of the young athlete's heart was reported until the age of 15. Active endurance athletes underwent eccentric remodeling, whereas a dynamic switch to concentric remodeling took place between the ages of 15 to 18 years of age [22]. However, it is not clear as to what the effect on the morphology and function of the child's heart would be due to different training approaches, including ones that incorporate intense training. The training load on young endurance athletes is an important unanswered question. The increasing competitiveness in sports has led to a trend of athletes training for endurance events at younger and younger ages. This shift has led to more intense training regimes, including long-distance running, swimming, and bike rides being incorporated into the regular routines of younger athletes looking to compete in these disciplines. Additionally, athletes are also being pushed to compete in multiple endurance events, when previously only one or two were the norm. The quantity and intensity that should be included in the training of young athletes, so that the development of their heart is optimized creating conditions for a healthy, strong

routines of younger athletes looking to compete in these disciplines. Additionally, athletes are also being pushed to compete in multiple endurance events, when previously only one or two were the norm. The quantity and intensity that should be included in the training of young athletes, so that the development of their heart is optimized creating conditions for a healthy, strong heart in the future and hence better performance, is a point that needs addressing. The notion that intense training at an early age could cause cardiac limitations so that the future of the child in sports would be jeopardized is also of great interest and with no rm answers so far. It is important to assess young athletes regularly for their cardiac adaptations due to training in order to identify whether parameters of training, like volume and intensity, result in detrimental physiological and/or structural changes. We are not aware of any study that examined the effects of different training approaches on the morphological and functional aspects of a child's heart. Therefore, the rst aim of this study was to investigate the cardiac functional and morphological adaptations, as well as the performance changes of children aged 12 to 14 brought about by two endurance training protocols. Furthermore, in order to assess whether morphological and functional development of the heart precedes, and performance improvement follows with some delay, a correlation analysis was performed between performance, functional and morphological characteristics following endurance training. 2. Material and Methods 2.1. Participants Thirty healthy untrained children (16 boys and 14 girls) participated and were ran- domly divided into 3 groups. We used the sealed envelope method of randomization to eliminate any potential bias and to ensure that the results of the trial are free from bias and accurately re ect the effectiveness of the intervention studied herein. The rst group consisted of 5 girls and 5 boys (Age = 12.4 0.7 years), served as the control group and did not follow a structured training protocol. The second group consisted of 6 girls and 4 boys (Age = 12.8 0.6 years) and these participants trained at an

and accurately re ect the effectiveness of the intervention studied herein. The rst group consisted of 5 girls and 5 boys (Age = 12.4 0.7 years), served as the control group and did not follow a structured training protocol. The second group consisted of 6 girls and 4 boys (Age = 12.8 0.6 years) and these participants trained at an intensity corresponding to the ventilatory threshold (ventilatory threshold training group, VTT): an intensity that is considered moderate. The third group consisted of 3 girls and 7 boys (Age = 13.1 0.3 years) and trained at 120% of VO2max (intensity training group, IT): an intensity that is considered highly intense. The rationale behind choosing these two intensities was

Sports2023,11, 157 3 of 13 to assess whether the intense training would lead to compromised cardiac remodeling. FigureSports 2023, 11, x FOR PEER REVIEW 3 of 13 ventilatory threshold (ventilatory threshold training group, VTT): an intensity that is con- sidered moderate. The third group consisted of 3 girls and 7 boys (Age = 13.1 ± 0.3 years) and trained at 120% of VO2max (intensity training group, IT): an intensity that is consid- ered highly intense. The rationale behind choosing these two intensities was to assess whether the intense training would lead to compromised cardiac remodeling. Figure 1 shows the CONSORT diagram of the study. Figure 1. CONSORT flow chart. Figure 1.CONSORT ow chart. Inclusion criteria for the participants were: (1) Healthy, 12–14 years of age, and (2) not participating in an organized sporting activity. Furthermore, none of the participants were on any prescribed medication and were able to cope with the strain of the experimental procedure. All participants and their parents declared their consent both orally and in writing after they were informed about the investigation process. Four participants dropped out because they could not follow the training (2 in VTT and 2 in IT), 6 participants

Sports2023,11, 157 4 of 13 developed lack of interest towards training (2 in VTT and 4 in IT) and 20 participants stopped training due to time constraints (6 in VTT and 4 in IT). No injuries were reported during the training period. The procedures followed were in full agreement with the Declaration of Helsinki of 1975, as it was revised in 2000, while an approval by the Ethics Committee of the University was provided (protocol number 505). 2.2. Experimental Design Participants visited the laboratory two times prior to the training intervention. During the rst visit, the anthropometric characteristics and their biological age was recorded using the Tanner method, an echocardiographic study was conducted, their maximal oxygen uptake (VO2max) was measured and their ventilatory threshold (VT) was determined. During the second visit, the time in velocity at VO2max (vVO2max) until exhaustion (Tlim) was determined. Measurements were obtained with a difference of at least 48 h. The same procedures were followed after the training intervention. Participants were instructed to refrain from participation in physical activity or exercise 48 h before the measurements. The research design is shown in Figure.Sports 2023, 11, x FOR PEER REVIEW 4 of 13 Inclusion criteria for the participants were: (1) Healthy, 12–14 years of age, and (2) not participating in an organized sporting activity. Furthermore, none of the participants were on any prescribed medication and were able to cope with the strain of the experi- mental procedure. All participants and their parents declared their consent both orally and in writing after they were informed about the investigation process. Four participants dropped out because they could not follow the training (2 in VTT and 2 in IT), 6 partici- pants developed lack of interest towards training (2 in VTT and 4 in IT) and 20 participants stopped training due to time constraints (6 in VTT and 4 in IT). No injuries were reported during the training period. The procedures followed were in full agreement with the Dec- laration of Helsinki of 1975, as it was revised in 2000, while an approval by the Ethics Committee of the University

VTT and 4 in IT) and 20 participants stopped training due to time constraints (6 in VTT and 4 in IT). No injuries were reported during the training period. The procedures followed were in full agreement with the Dec- laration of Helsinki of 1975, as it was revised in 2000, while an approval by the Ethics Committee of the University was provided (protocol number 505). 2.2. Experimental Design Participants visited the laboratory two times prior to the training intervention. Dur- ing the first visit, the anthropometric characteristics and their biological age was recorded using the Tanner method, an echocardiographic study was conducted, their maximal ox- ygen uptake (VO2max) was measured and their ventilatory threshold (VT) was deter- mined. During the second visit, the time in velocity at VO2max (vVO2max) until exhaus- tion (Tlim) was determined. Measurements were obtained with a difference of at least 48 h. The same procedures were followed after the training intervention. Participants were instructed to refrain from participation in physical activity or exercise 48 h before the measurements. The research design is shown in Figure 2. Figure 2. Schematic representation of the experimental design. Upward arrows indicate cardiopul- monary exercise measurements and downwards arrows indicate echocardiographic measurements. 2.3. Anthropometric Measures Body mass, height and fat percentage of seven skinfolds were evaluated. All the an- thropometric assessments were performed according to the instructions of the American College of Sports Medicine (ACSM, 9th Edition) in the morning, following an overnight fast. Body mass was measured to the nearest 0.05 kg (Seca alpha 770 scales, Vogel & Halke Hamburg, Germany) while the participants were lightly dressed and barefoot. Standing height was measured to the nearest 0.5 cm (Seca Stadiometer 208; Vogel & Halke Ham- burg, Germany). Percentage body fat was estimated from 7 skinfold-thickness measures (average of two measurements of each site) by using a Harpenden caliper (John Bull, St. Albans, United Kingdom). The Siri skinfold-thickness equation was used to calculate body fat (ACSM, 9th Edition). The biological age of the participants was assessed in the lab by a trained medical examiner of the participants Tanner s sexual maturation

was estimated from 7 skinfold-thickness measures (average of two measurements of each site) by using a Harpenden caliper (John Bull, St. Albans, United Kingdom). The Siri skinfold-thickness equation was used to calculate body fat (ACSM, 9th Edition). The biological age of the participants was assessed in the lab by a trained medical examiner of the participants Tanner s sexual maturation stages. Figure 2. Schematic representation of the experimental design. Upward arrows indicate cardiopul- monary exercise measurements and downwards arrows indicate echocardiographic measurements. 2.3. Anthropometric Measures Body mass, height and fat percentage of seven skinfolds were evaluated. All the anthropometric assessments were performed according to the instructions of the American College of Sports Medicine (ACSM, 9th Edition) in the morning, following an overnight fast. Body mass was measured to the nearest 0.05 kg (Seca alpha 770 scales, Vogel & Halke Hamburg, Germany) while the participants were lightly dressed and barefoot. Standing height was measured to the nearest 0.5 cm (Seca Stadiometer 208; Vogel & Halke Hamburg, Germany). Percentage body fat was estimated from 7 skinfold-thickness measures (average of two measurements of each site) by using a Harpenden caliper (John Bull, St. Albans, United Kingdom). The Siri skinfold-thickness equation was used to calculate body fat (ACSM, 9th Edition). The biological age of the participants was assessed in the lab by a trained medical examiner of the participants' Tanner's sexual maturation stages. 2.4. Maximal Oxygen Uptake Maximal oxygen uptake (VO2max) was assessed through the use of a protocol of gradual speed increase on a treadmill (TechnoGym Run Race, Rome, Italy). The procedure was carried out after a 12 min warm-up (8 min of relaxed jogging at 60% of maximal heart rate, and 4 min of stretching). The initial speed was set to 8 km h 1 . Then, increased by 1 km h 1 for every one minute. The O2and CO2rates in the exhaled air were measured through a gas analyzer (VO2000, Sensormedics, Yorba Linda, CA, USA), which was cali- brated before each test using standard gases of known concentration, while the heart rate

km h 1 . Then, increased by 1 km h 1 for every one minute. The O2and CO2rates in the exhaled air were measured through a gas analyzer (VO2000, Sensormedics, Yorba Linda, CA, USA), which was cali- brated before each test using standard gases of known concentration, while the heart rate

Sports2023,11, 157 5 of 13 was recorded with a heart-rate monitor (Polar S410, Kempele, Finland). The volumes of exhaled gases were recorded (breath by breath) every 20 s. Criteria used to determine VO2max were: (i) participants' exhaustion, (ii) a <2 mL kg 1 min 1 increase in VO2with an increase in work rate, (iii) a respiratory exchange ratio greater than or equal to 1.10, (iv) a heart rate within 10 bpm of the theoretical maximum heart rate (220-age). The velocity at maximal oxygen consumption (vVO2max) and the velocity at ventilatory threshold (VT) were recorded. The VT was determined by the V-slope method [27], while vVO2max was de ned according to Billat et al. [28]. 2.5. Time to Exhaustion In order to determine time to exhaustion (retention time in vVO2max until exhaustion, Tlim) participants, had to reach their individual vVO2max, in 30 s after starting their examination from a state of inertia. Once a subject reached the vVO2max, the measurement of Tlim began. Timing lasted until the subject was volitionally exhausted. 2.6. Echocardiogram The echocardiography examinations were conducted by a registered diagnostic medi- cal sonographer with more than ve years experience, using the clinical imaging system (Hitachi Aloka ProSound Alpha 6, Hitachi Aloka Medical Ltd., Tokyo, Japan) and 5-MHz transducer. Standard echocardiographic views (parasternal, apical, and substernal) were obtained. All images were reviewed to con rm that parasternal long axis (PSLA) paraster- nal short axis (PSSA), four-chamber (4-CH), and two-chamber (2-CH) views were acquired with at least three complete cardiac cycles, and images were obtained at a frame rate of 50 to 80 frame/s. 2-CH and 4-CH images were collected and yielded comparable results. How- ever, due to time limitations imposed by the children, it was not possible to acquire these images from all participants. All the recorded views from the parasternal long axis (PSLA) were reviewed to select the most suitable with minimal heart rate variability. Guidelines by the European association of cardiovascular imaging were adhered to when acquiring and analyzing the recorded images. The sonographer was blinded to the characteristics of the participants, the group allocation and time point

The study examines heart adaptations in children from different endurance training methods.